T helper cell subtypes Th1 and Th2 cells arise from a common precursor cell in response to triggering through the T cell receptor and cytokine receptors for IL-12 or IL-4. This leads to activation of complex signaling pathways. Disturbances in the balance between type 1 and type 2 responses can lead to certain immune-mediated diseases such as asthma, allergy and certain autoimmune diseases (1-3). Thus, it is important to understand how Th1 and Th2 cells are generated.
Th1 and Th2 cells are known to originate from naïve CD4+ precursor cells (Thp) after antigenic activation through the T cell receptor (TCR) and co-stimulatory molecules in a suitable cytokine milieu. The main cytokines orchestrating Th1 and Th2 development are IL-12 and IL-4, respectively. Triggering of the TCR and cytokine signaling leads to activation of complex, and to a large extent poorly understood, downstream signaling networks that finally result in maturation of the effector Th1 and Th2 cells (4, 5). IL-12 (and in human also IFNα) induces the Th1 type response by activating the Signal Transducer and Activator of Transcription 4 (STAT4) mediated signaling pathway (6-8). Some other cytokines such as IFNγ and IL-18 can also promote Th1 responses, especially in combination with IL-12. Th2 differentiation is induced by IL-4 through the STAT6 signaling pathway (9-12). GATA binding protein 3 (GATA-3), avian musculoaponeurotic fibrosarcoma (v-maf) AS42, oncogene homolog (c-maf) and T-box expressed in T cells (T-bet) are also among the most important factors regulating the early polarization of Th2 and Th1 cells respectively (11-16).
Another important cytokine involved in Th1 and Th2 differentiation is TGFβ. This immunosuppressive cytokine exhibits pleiotrophic activities in various cellular processes and, importantly, can suppress the differentiation of CD4+ cells into the Th1 and Th2 subtypes (17). However, similar to IL-12 and IL-4, the target genes and details of TGFβ downstream signaling are not clear.
To provide a basis for understanding the mechanism of action and molecular networks involved in the signaling of these cytokines, the early phase leading to polarization of Th1 and Th2 cells in the presence and absence of TGFβ was examined. As a result, genes differentially regulated in the cells induced to polarize to Th1 and Th2 subtypes in human were identified. Importantly, to our knowledge 77 of these genes have not been previously described to be involved in Th1 and Th2 cell differentiation. In addition, we have further clarified which of the genes involved in the early polarization of human Th1 and Th2 cells are targets of IL-12 and IL-4 regulation and which of them are also targets of immunosuppressive TGFβ. Wild type mouse cells or STAT6-knockout cells were used to further clarify the mechanism how IL-4 regulates gene expression through STAT6 signaling. These genes newly identified genes involved in Th1 and Th2 differentiation serve as therapeutic targets in achieving an appropriate balance between Th1 and Th2 responses.